A number of different applications of catadioptric imaging systems for far-field and near-field interferometric confocal microscopy have been described such as in various of the commonly owned U.S. Patents, U.S. Provisional Applications, U.S, Non-Provisional Applications listed above.
In some of the catadioptric imaging systems that are described in those patents and patent applications, conjugate images of pinhole arrays in or on a substrate, pinhole array beam combining beam-splitters, and/or arrays of detector pixels are located in two-dimensional planes with off-axis aberrations compensated to extend the field of view for diffraction limited imaging by the use of concentric dispersive elements.
In contrast, at least some of the embodiments described herein reduce the effects of off-axis aberrations by the use of conjugate arrays of orthogonal slits located on the sagittal and tangential surfaces in either the object space to generate a diffraction limited spot on a measurement object and/or in the image space of catoptric imaging systems to obtain diffraction limited information about a spot on a measurement object.
Also in certain of the applications of catadioptric imaging systems that are described in the above-identified patents and patent applications, a beam-splitter is incorporated in generating an image of an object with zero optical aberrations for a measurement object located on the optic axis of the imaging system. The beam-splitter is located at an interface between relatively thick optical elements of the catadioptric imaging systems. The optical elements contribute as well as compensate off-axis aberrations and cause a significant portion of optical paths in the catadioptric imaging systems to comprise a refractive medium such as fused silica or CaF2.
In contrast, at least some of the embodiments described herein involve use of a thin beam-splitter in catoptric imaging systems to generate an image of a measurement object with zero or substantially zero optical aberrations for an object located on the optic axis of the imaging system configured for operation in the IR to EUV. The use of the thin beam-splitter reduces the magnitude of both the off-axis and on-axis aberrations that may or may not require subsequent compensation, increases the field of view that may be used, and reduces the optical path length in a transmitting refractive medium which is particularly important when working in either the IR to the EUV.
Also in certain of the applications of catadioptric imaging systems that are described in the above-identified patents and patent applications, tight tolerances are generally placed on the manufacture of optical elements. In addition to the tolerances normally encountered in designing a diffraction limited imaging system, there are additional tolerances imposed in interferometric confocal and non-confocal microscopy applications. The additional tolerances are for example on surfaces of certain elements with respect to radii of curvature and on relative locations of centers of curvature of the surfaces of the certain elements.
The additional tolerances lead to improved performance of a catoptric imaging system, e.g., with respect to increasing the average intensity of desired images by a factor of approximately 2 and reduced intensity of spurious beams by one or more order of magnitudes, and in addition make it possible to realize interferometric reduction of background fields. The interferometric reduction of background fields leads to a reduction of statistical errors. The increase in intensity of desired images and the reduction of statistical errors lead to an increase in signal-to-noise ratios and to a concomitant increase in throughput of a metrology tool using the catoptric imaging system. The interferometric reduction of background fields further leads to a reduction of systematic errors. A consequence of the reduction of systematic errors is a reduction of the computational task required to invert arrays of measured interference signal values to a multi-dimensional image of a measurement object.
At least some of the above-identified U.S. Patents, U.S. Patent Applications, and U.S. Provisional Patent Applications further teach the use of adaptive catoptric surfaces in a catoptric or catadioptric imaging system. The use of adaptive catoptric surfaces in a catoptric imaging system makes it possible to relax tolerances on the surface figures of elements, to relax tolerances on locations of surfaces of the elements in the catoptric imaging system, and to compensate for certain optical aberrations such as may be introduced by the pellicle or aperture-array beam-splitter. The factor by which the tolerances may be relaxed on the surface figures is of the order of 5 for certain of the elements. The use of adaptive catoptric surfaces in a catoptric imaging system further makes it possible to introduce a vertical or lateral scan of a measurement object or substrate being imaged at slew rates higher then possible and/or practical when the vertical or lateral scan must otherwise be introduced either by translations of an entire catoptric imaging system and associated optics and detector systems or translations of the measurement object or substrate, e.g., a 300 mm wafer, and the measurement object or substrate support system.
Certain of the above-identified cited U.S. Patents, U.S. Patent Applications, and U.S. Provisional Patent Applications further teach the replacement of a beam combining beam-splitter in an interferometric imaging system with an interface comprising a thin fluorescent layer or array of thin fluorescent spots.
The cited U.S. Patents, U.S. Patent Applications, and U.S. Provisional Patent Applications also teach the use of an N-dimensional bi- and quad-homodyne detection methods.
The use of multi-element adaptive catoptric surfaces in catoptric imaging systems also makes it possible to compensate for optical aberrations such as may be introduced by a pellicle or aperture-array beam-splitter or such as introduced when imaging a plane section of a substrate wherein one or more plane refracting surfaces are located for example in the object space of the catoptric imaging system near and in front of the plane section of the substrate. The compensation of the optical aberrations corresponds to the conversion of one or more spherical catoptric surfaces to one or more aspherical catoptric surfaces.
As described herein, the replacement of a beam combining beam-splitter in interferometric imaging system with a beam combining thin fluorescent layer or interface or with an array of thin fluorescent spots for operation in the UV to EUV impacts on the performance specifications required of optical elements of the interferometric imaging system and/or detector that follow the beam combining function to achieve a certain end use performance. The thin fluorescent layer or array of fluorescent spots, e.g., lumogen, absorbs light at one wavelength, e.g., the EUV, and emits light at a longer wave length, e.g., in the visible, to generate an optical interference signal. The optical interference signal is subsequently converted to an electrical interference signal when the longer wavelength light is detected by a detector. Thus, there is a concomitant reduction in the required performance specifications of the optical elements because the optical elements serve only to transmit beams and generate optical images at the longer wavelength instead of at the shorter wavelength beam in the UV to EUV. The shorter wavelength beam that is absorbed is a mixed beam which comprises a measurement beam component and a reference beam component in the same polarization state.
In the case where a beam-splitter is used for the beam combining function, the measurement beam component and the reference beam component of the combined beam may have subsequent to the beam-splitter different paths in the optical elements which introduce the possibility of non-common path phase errors. The possibility of non-common path phase errors is not present when a thin fluorescent layer or array of fluorescent spots serves the beam combining function.
When the shorter wavelength beam has a wavelength in the UV to EUV and a thin fluorescent layer or array of fluorescent spots serves the beam combining function, there is a significant change in the required performance of the detector because it has to serve to only detect the longer wavelength optical beam instead of the shorter wavelength mixed beam. The advantage of at least some of the embodiments described herein with respect to the reduction on the required performance specifications of the optical elements and the detector is valid for measurement and reference beams comprising UV to EUV wavelengths.
The implementation of the N-dimensional bi- and quad-homodyne detection methods make it possible to extend the advantages of the bi- and quad-homodyne detection methods for measuring conjugated quadratures of fields jointly to homodyne methods for measuring conjugated quadratures of fields when measuring jointly N different properties of the fields.